As is known, there is an increasing need for protection against the penetration of contaminants into a sheltered space, given the threat of military attacks and acts of terror using nuclear, chemical or biological (NBC) means of warfare. Such protection is provided by the use of shelters and protected spaces located inside buildings (“hard” shelters), or tent structures (“soft” shelters). Hard shelters are provided with professional NBC air filtration systems including blowers, which create an overpressure inside the protected space and provide cleaned and filtered air. The overpressure is required in order to ensure a clear direction of airflow from inside the protected space to the outside atmosphere, through ever-present leaks in the walls of the shelter, the door and the window seals. Use of this method makes sure that no contaminated air will penetrate into the protected space.
In a typical soft-type shelter, clean, NBC-filtered air is pushed by blowers into the tent, with excess air passing to the outside through leaks or specially created holes in the walls of the tent. The disadvantage of such an arrangement is that, during times when no fresh air is pumped into the protected space, e.g., during a blower breakdown, the essential overpressure immediately drops, with two dire results: contaminated air can enter the protected space via the air exit holes, and, if the protected space is in fact an inflated structure, the internal liner bubble may collapse, trapping inside all of the persons and goods to be protected. If these structures had closable valves, none of the above failures would be able to occur and the system would remain intact for a prolonged period of time until the clean air supply resumes and creates the required overpressure.